The preferred embodiment relates to a pneumatic amplifier which generically is intended to comprise a supply air valve and an exhaust air valve. The supply air valve is connected with a pneumatic supply which supplies, for example, a supply pressure of 6 bar. Through a pneumatic connection, the pneumatic amplifier receives a pneumatic input control signal, which is generated, for example, by an electro-pneumatic converter. The electro-pneumatic converter can be, for example, a flapper nozzle arrangement or a modulated switching element, which is controlled pulse width modulated (PWM: “Pulse Width Modulation”). Typically, such electro-pneumatic converters are moved magnetically or by a piezo-electrical component.
The amplifier serves for increasing the air flows for venting and bleeding a volume. The amplifying degree depends on the supply pneumatics and the valve construction of the supply air valve or the exhaust air valve, respectively. As is known, because of its moveable valve member, the supply air valve can continuously open or cut off a pneumatic connection between signal input and signal output of the pneumatic amplifier. Furthermore, a diaphragm is coupled force- or form-locking to the valve member of the supply air valve. On its outside facing towards the signal input of the amplifier, the diaphragm is exposed to the input control signal pressure of the electro-pneumatic converter.
The exhaust air valve is provided with a bleed output for bleeding of the pneumatic amplifier. Typically, the exhaust valve has a valve member for continuously opening or cutting off a pneumatic connection between the signal output of the supply air valve or the volume, respectively, the pressure of which is to be controlled, and the bleed output.
For illustration of the functionality of a conventional pneumatic amplifier, reference is made to the attached FIGS. 1 and 2, which represent the schematic diagrams of the cross section of the amplifier in two states. FIG. 1 shows a venting or air-powering state of the known amplifier, and FIG. 2 shows the bleeding or exhausting state of the pneumatic amplifier.
The pneumatic power amplifier a has a pneumatic supply input b at which the supply pressure Pv is applied. Furthermore, the pneumatic amplifier a has a bleed output c to which the atmospheric pressure Patm is applied. In addition, the pneumatic amplifier a has a control signal input d, through which the pneumatic input signal PS is received by an electro-pneumatic control unit (not shown). Finally, the pneumatic amplifier has a control signal output z.
A housing of the pneumatic amplifier a is divided into a supply chamber e, into which the supply input b runs, a working chamber f to which the control signal output z is allocated, a bleed chamber g with the bleed output c, and a control chamber h with the control signal input d. In a partition wall between the supply chamber e and the working area f, an opening i is formed which defines a valve seat for a first conical supply air valve member k. Between the working chamber f and the bleed chamber g, a first moveable diaphragm wall l is implemented which has a bleed or exhaust opening t which defines a valve seat for an exhaust air valve member n. The bleed chamber g is separated from the control chamber h by a second moveable diaphragm wall o, which is spring-biased. Between the control chamber h and the working chamber f, a bypass line q is provided, which can be continuously opened or closed by a throttle pin r. In this manner, the control behavior of the amplifier, in particular of the switching operation between bleeding and venting, can be optimized by setting a bypass flow md from the control chamber h into the working chamber f.
During the venting process (FIG. 1), the pneumatic amplifier a, via the control signal input b, receives the pneumatic control signal PS, by means of which the second diaphragm wall o is moved downwards against the pre-tension of the spring, wherein by means of a rigid coupling of the first diaphragm wall l to the second diaphragm wall o, the exhaust air valve member n is brought in contact with the valve seat to close the bleed opening t. Moreover, the supply air valve member k is moved downwards due to the structural coupling of the exhaust air valve member m with the supply air valve member k, whereby the valve seat of the opening i is released, and a supply flow passing through the opening i into the working chamber f is allowed. In this manner, a control flow mp towards a pneumatic actuator (not shown) can be released.
During bleeding or exhausting (FIG. 2) the pneumatic amplifier a, there is no control pressure PS in the control chamber h, whereby the spring-biased second diaphragm wall o is moved upwards, releasing the valve seat of the bleed or exhaust opening t, and, at the same time, the spring-biased supply air valve member k is moved towards the valve seat of the opening i. In this manner, a bleed flow menti through the pneumatic amplifier a towards the bleed output c is possible.
The known pneumatic amplifier has the disadvantage that it requires a quite complex structure and hence high manufacturing cost for its production. It is particularly difficult to configure the switching function from venting to bleeding and vice versa by means of the dimensioning and design of the mechanical components. For this, it is important to generate a defined operation change upon switching the pneumatic amplifier, in particular by means of the adjusting pin.
Typically, the pneumatic amplifier is used for a pneumatic control system for controlling a control valve of a process plant, which is known, for example, from EP 0 884 667 A1. According to this, the pneumatic amplifier is comprised of two supply air and bleed valves connected in parallel, each of them connected with a pre-control unit, in particular with a electro-pneumatic converter.
The bleed unit is designed for a “negative” amplification, that is, for bleeding, and the supply air valve for “positively” amplifying the pneumatic control signal. The bleed control signal or the amplified signal, respectively, is to be transmitted to a pneumatic actuator. This arrangement, in pairs of two independent pre-control units with a separate control and each with a downstream supply air/bleed valve, is complex and cost-intensive, wherein the high number of parts results in an unfavorable high failure probability of the system, which during operation has to meet high safety standards.
DE 42 40 802 C2 discloses an electro-pneumatic converter to which a pneumatic amplifier is connected which is comprised of a supply air valve and an exhaust air valve. To both valves a pneumatic pre-control signal is applied.
Each of the valves is comprised of a double-diaphragm arrangement with different tappets between the two diaphragms and from the second diaphragm to the valve. The active areas of both diaphragms are here considerably different in size so that the output pressure reacts as little as possible on the control diaphragm. A pressure building up between the diaphragms, just because of its influence during temperature changes, would act as a disturbance. According to the typical design, the chamber located between the diaphragms is bled to the atmosphere to avoid this disturbance. The mechanical construction of this amplifier arrangement, however, is very complex, and the assembly is difficult and therefore overall expensive.